The present invention pertains to the field of servo-mechanical actuators and, in particular, to the high precision and high speed positioning of objects using actuators.
Electromechanical and servo-mechanical systems make extensive use of a very wide range of actuators. Magnetic actuators in particular find wide industrial use as these devices lend themselves to control via the use of appropriately controlled electric current through suitably designed electromagnets. Applications range widely. While much effort has gone into balancing systems for valves, levitation devices, and xe2x80x9cfrictionlessxe2x80x9d bearings, it is the high speed and high accuracy arena, such as the fields of data storage on various magnetic and optical media, that has provided an impetus for the development of the technology. In these applications magnetically actuated reading and writing heads have to be positioned at very high speeds to high accuracies over data tracks and the vertical positioning of the heads has to be controlled to maintain either a magnetic writing distance or an optical focal distance.
With the more recent advent of microlithographic technology and, more particularly, the development of microelectromechanical (MEMS) devices, it has also become possible to viably employ electrostatically driven actuation devices. The fact that sufficiently high actuating electrostatic field strengths may be attained using practical voltage levels at the characteristically small inter-electrode distances, lies at the root of this development. More recently magnetically actuated microelectromechanical devices have also been described.
One specialized application field for actuators that has seen much growth in recent times is the digital-on-press developments within the field of lithographic printing. In this field, image data is written directly to a blank lithographic plate using a laser head equipped with modulated laser arrays.
One of the particular challenges in this arena is the need for very precise control of the distance between the focusing lens and the printing plate surface. To establish this control, the focusing lens is affixed to the moving member of an actuator. This moving member is referred to herein as a plunger. Data is written at very high speeds in these systems and the focusing lens has to maintain a precise xe2x80x9cflying heightxe2x80x9d in this process. Because of the large lateral distances traversed in this application compared with typical data storage devices, the actuators have to maintain this accurate separation while the plunger of the actuator traverses over a considerable stroke length to allow the focusing lens to xe2x80x9cfollowxe2x80x9d the variation of the printing plate or medium.
In the most simple incarnation of a linear actuator, a force, either magnetic or electrostatic, is applied to the plunger. The plunger is made subject to a restoring force. In this most simple case, this force is provided by a spring with a simple spring constant. This results in the restoring force being essentially linear in the sense that the force provided by the spring is proportional to the linear displacement of the plunger. It is possible to eliminate the restoring force if a bi-directional actuator, such as a voice-coil, is used.
The actuating force, on the other hand, being magnetic or electrostatic in nature, is inherently non-linear. In particular, it is known to those skilled in the art of magnetic actuators in particular, that, in principle, the net force on the plunger in a magnetic actuator is related to the square of the magnetizing current and inversely related to the square of the magnetic gap (also called the air gap) between the magnetic member of the plunger and the fixed driving electromagnet. In practice this relationship is even more complex because, amongst other reasons, saturable magnetic elements are employed to manipulate the behavior of the actuator. The relationship between gap and force also depends upon the geometry of the actuator.
Because of these highly non-linear relationships, a significant problem arises in linearizing the net output force of the moving member of a magnetic actuator in response to an applied force command; i.e., to obtain a plunger force proportional to the input signal in order to establish adequate control over the actuator.
In providing linear force control on the plunger of magnetic actuators, previous control techniques have included flux feedback, force feedback arid current/gap feedback methods. In the case of electrostatic actuators there has been comparatively little described in respect of means to control such actuators beyond simple two-state devices. This dearth of practical analog electrostatic actuator devices is related partly to the nature of the applications that employ them, but also in particular to the difficulty in controlling them in view of the non-linear actuation forces.
In the flux feedback approach used in magnetic devices in particular, the magnetic flux experienced by the plunger is monitored continuously by a sensor, typically a Hall-effect device, and this information is fed back to the control system. Via a wide variety of electronic and computing means, an appropriate compensating current is then applied to the electromagnet driving the device.
One of the drawbacks of this approach for actuators working at high speeds over small stroke lengths is the fact that it is extremely awkward to have a sensor occupying any significant fraction of the magnetic gap (air gap). Recessing the sensor, either into the plunger or into the pole piece of the electromagnet, can cause the sensor to measure a field-strength not entirely representative of the field experienced by the plunger. The relationship between measured field and force on the plunger is therefore perturbed.
The force feedback approach, which is in concept a variation on the flux-feedback technique, incorporates a force sensor in a closed-loop configuration to linearize the net forces on the plunger. The plunger is physically tied to the payload through the force sensor. Any force exerted on the plunger is transmitted through the force sensor to the payload. Force sensors vary, but are typically quartz oscillator crystals, which vary an oscillator frequency in response to a tensile or compressive force. This frequency shift is then used to control the force on the plunger, by adjusting the magnetic flux density produced by the control electromagnet.
Among the drawbacks to this approach is the fact that force sensors capable of a high bandwidth and resolution required for low-level force control are costly, fragile, and require sophisticated support electronics. Further, because of their fragility, these sensors often require elaborate holding fixtures to protect them from damage.
As another approach to force linearization, current/gap feedback has been used. This technique is more common and utilizes the relationship between magnetizing current and air gap for a linear medium in an open force loop configuration. In this method, the force is controlled by employing the fact that the plunger force is nominally proportional to the square of the magnetizing current and inversely proportional to the square of the magnetic gap.
In this approach, any sensor capable of providing a signal proportional to gap position can be used. Previous applications have incorporated eddy current, capacitive, and inductive sensors. By employing both current and gap position sensors, the requirement for and disadvantages of a force sensor are eliminated. To remove the current non-linearity, various bias current techniques have been utilized. However, because of the open force loop configuration and square law relationships, both the position and current sensing signals, as well as squaring compensation circuitry, must be very accurate and linear over all operational conditions. Because of the nominally squared relationship, percentage force errors can be more than twice the percentage position and gap errors that cause them.
It is true in all the aforementioned situations, that the applied plunger force is directly determined by the magnetic field established in the magnetic gap due to the electromagnet coil currents. A linear relationship between the input coil current and plunger movement is therefore ideally required for classic feedback and control systems. The term linear, as used here, means that the differential equations describing the behavior of the actuator are linear. Therefore, non-linearities due to hysteresis and saturation between the magnetic flux and input coil current can seriously impact actuator performance and reduce the applicability/effectiveness of classical control systems algorithms.
The development of high-speed digital control systems has allowed the development of actuator control mechanisms based on pre-calibrated software lookup tables. The typical approach has been to calibrate the current through the electromagnet against displacement of the plunger. This non-linear relationship is then stored as a table of values, and the controller looks up the appropriate electrical current to apply through the electromagnet to balance the restoring force of the spring for the particular degree of displacement of the plunger. This current is applied and the plunger moves to this position of choice.
While this simple one-variable approach is adequate for slow systems, it does not adequately address the problems of high-speed systems where dynamic behavior is very important. These issues may be addressed in part by creating two-dimensional lookup tables that employ both actuator current and the plunger displacement as parameters, these require excessive digital memory and the overall solution becomes more expensive. A method is required that will address the dynamic or xe2x80x9csmall signalxe2x80x9d behavior of high-speed magnetic actuators without making excessive demands on processing power and memory.
In the field of electrostatic devices, actuators have much in common with audio speakers, and the advances in producing audio speakers with superlative frequency characteristics and dynamic range are relevant. However, these devices are limited to comparatively small movements at audio frequencies.
The advent of MEMS devices has made high frequency electromechanical devices possible. However, the micro-miniaturization makes complex device element arrangements difficult while field-saturable materials are less easy to incorporate and force-feedback is not practicable. There is therefore a particularly great need for a method to linearize the force gradient in electrostatic devices, particularly MEMS devices.
While some of the background gleaned from the discrete magnetic actuator devices is relevant, the existing approaches do not adequately address the unique problems facing the designer of an electrostatic microelectromechanical actuator.
Much attention has been devoted to microelectromechanical devices in various forms. Accelerometers, in particular, share many of the problems related to the design and manufacture of actuators. However, there is one critical aspect in which accelerometers have a significant advantage from which actuators do not benefit namely, the fact that the accelerometers intentionally keep the plunger stationary. Instead they measure the voltage or current required to maintain it in that state when the system into which the accelerometer is incorporated, is accelerated. The plunger therefore does not traverse a significant distance within the electrostatic field in the case of an accelerometer. In the actuator this traverse distance is one of the figures of merit of the device and needs to be as large as possible and as accurately controllable as possible.
It is an objective of the present invention to obtain improved dynamic performance at high displacement speeds and frequencies from an actuator driven with a substantially non-linear force by linearizing the relationship between the actuating impetus and the feedback signal through the separate and concurrent control of dynamic and static characteristics of the actuator.
It is a further objective of the present invention to address the linearization of the relationship between actuating impetus and feedback signal in the particular case of actuators where space is at a premium and additional feedback sensors are difficult to accommodate.
It is yet a further objective of the present invention to address the linearization of the relationship between actuating impetus and feedback signal in a microelectromechanical actuator where additional feedback sensors are particularly difficult to implement and the non-linearity in the actuating force has particularly detrimental consequences.
In accordance with the present invention, the control over a high-speed non-linear actuator is improved by linearizing the relationship between the actuating impetus and the feedback control signal via a method that employs the separate and concurrent control of the static and dynamic characteristics of the device without resorting to the use of force-feedback or field-strength feedback. The resonant frequency of the plunger of the actuator is manipulated during operation such as to maintain it at a substantially fixed optimal value. The method is particularly advantageous in devices where space is at a premium and force-feedback or field-strength feedback mechanisms are difficult to implement.